1. Field of the Invention
The present invention relates to a polymeric sensor having selective binding affinity for an analyte which is indicative of food spoilage. More particularly, this invention is directed to a polymer containing incorporated therein a macrocyclic transition metal complex which selectively binds a biogenic amine, whereupon the polymer undergoes a detectable color change and thereby indicates the extent of food spoilage. Selectivity of the polymer sensor may be enhanced through the process of molecular imprinting.
2. Description of the Related Art
Polymeric sensors are prepared by ionically or covalently attaching an atomic, ionic, or molecular site to a polymer that, upon association with a particular analyte, will exhibit a detectable change in a measurable physical property. Exemplary measurable physical properties include spectroscopic (i.e., electronic absorbance or luminescence), electrochemical, or magnetic properties. The change in the physical property then provides a probe for the presence or absence of the associated analyte that can be measured using appropriate instrumentation or by direct observation. The polymer provides a support matrix that serves to immobilize the sensor sites and provide a localized density of the sensor sites as a means of optimizing detection of the analyte.
Molecular imprinting essentially involves making a polymer cast of a target molecule. The process of making the polymer cast involves dissolving the target molecule to be imprinted in a suitable solvent. Normally, a co-monomer, cross-linking monomer and a polymerization initiator are added to the reaction mixture. Radiation (photochemical or ionizing) or thermal energy is then applied to the reaction mixture to drive the polymerization process, ultimately resulting in the formation of a solid polymer. The resulting polymer may be processed using conventional polymer processing technologies, assuming those processes do not alter the structure of the molecularly imprinted sites. The imprinted molecule is extracted using methods appropriate for dissociating the target molecule from the polymer. Details of target molecule dissociation from the polymer are dependent upon the nature of the chemical interaction between the target molecule and the polymer binding site. The polymer dissociated from the target molecule possesses binding sites optimized for the structural and electronic properties of the target molecule. Mosbach U.S. Pat. No. 5,110,833 describes the preparation of synthetic enzymes and synthetic antibodies by molecular imprinting techniques.
The advantages to polymer sensors, including molecularly imprinted polymer (MIP) sensors, include their high molecular sensitivity, robustness, long storage life without losing their sensitivity, low cost, and ease of production. It is for these reasons that MIPs are being utilized in many industries. Current biomedical applications include drug monitoring devices and implants to monitor glucose levels.
Tetraazacyclotetradecanenickel(II), referred to as Ni(cyclam)2+, due to its geometric properties and resulting large ligand field stabilization energy, exists in water predominantly as a square planar, four-coordinate, yellow-colored complex (Connolly, P. J.; Billo, E. J. Inorg. Chem. 1987, 26, 3224-3226). Upon addition of ethylene diamine, aqueous solutions of Ni(cyclam)2+ turn from yellow to a blue/violet color, the origin of which has been attributed to cis coordination of the diamine to two adjacent coordination sites of the central nickel(II)ion (Billo, E. J. Inorg. Chem. 1984 23, 2223-2227). The availability of the cis coordination sites on the nickel(II) ion is made possible by folding of the cyclam ligand, a process that is known to involve isomerization of the nitrogen donor set (Barefield, E. K.; Bianchi, A.; Billo, E. J.; Connolly, P. J.; Paoletti, P.; Summers, J. S.; Van Derveer, D. G. Inorg. Chem. 1986, 25, 4197-4202). The change in geometry from four-coordinate in the absence of the diamine to six-coordinate in the presence of the diamine results in a change in the ligand field splitting of the central nickel(II) ion, and the blue/violet color of the product. See FIG. 1.
A Ni(II) complex related to Ni(cyclam)2+, 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecanenickel(II), referred to as Ni(Me2[14]aneN6)2+, possesses a 14-membered skeleton structure, although the propylene bridges of cyclam are now replaced with tertiary amines. This compound is known to exist predominantly in the yellow form in aqueous solution and has properties similar to those of Ni(cyclam)2+ (Suh, M. P.; Kang, S.-G. Inorg. Chem. 1988, 27, 2544-2546). A related compound, with 2-hydroxyethyl groups at the 1 and 8 positions in place of the methyl groups, has recently been found to react with ethylene diamine to yield a cis-hexacoordinate nickel(II) complex (Xiang, H.; Lu, T.-B; Chen, S.; Mao, Z.-W; Feng, X.-L; Yu, K.-B. Polyhedron 2001, 20, 313-319).
Meat spoilage occurs as bacteria begin to grow unchecked following respiration and circulation cessation at the time of slaughter. One of the markers of meat spoilage is the decarboxylation of free amino acids on and in the meat by enzymes released by spoilage microorganisms. Two of these products, putrescine and cadaverine, are particularly distinctive in odor, correlate well with surface bacterial counts, and are widely used to evaluate meat freshness both by trained meat inspectors and the individual consumers. Another product, histamine, is of interest due to its apparent ability to potentiate histamine intoxication, a form of food poisoning associated with the consumption of spoiled fish.
Accordingly, there is a need for a robust sensor device which accurately, simply and rapidly detects the presence of biogenic amines in food products.
The present invention provides a polymeric sensor which exhibits selective binding affinity for biogenic amine and undergoes a detectable change in absorption or emission of electromagnetic radiation upon exposure to biogenic amine. Upon contact with biogenic amine, such as putrescine (1,4-diaminopentane), cadaverine (1,5 diaminopentane) and/or histamine (5-imidazole-ethylamine), the polymer typically undergoes a detectable color change from yellow to blue/violet. The polymer is formed by the steps comprising:
(A) providing a four-coordinate polyazamacrocyclic transition metal complex containing one or more polymerizable moieties;
(B) copolymerizing the transition metal complex of step (A) with monomer and optional crosslinking agent to form a polymer which exhibits selective binding affinity for biogenic amine and undergoes a detectable color change when the target molecule binds thereto.
In another embodiment, in step (A) the four-coordinate polyazamacrocylclic transition metal complex is reacted with a target molecule comprising at least one biogenic amine to provide a reaction product possessing five or six-coordinate geometry. This reaction product, monomer and optional crosslinking agent are then copolymerized using conventional techniques to produce a polymer. Thereafter, the target molecule is removed from the polymer to provide a molecularly imprinted polymer which exhibits selective binding affinity for the target molecule and undergoes a detectable color change when the target molecule binds thereto.
The color change may be in the UV, visible or near infrared region or some combination thereof and will occur to an extent dependent upon the concentration of biogenic amine to which the polymer is exposed. For food spoilage applications, significant color change will take place when the polymer is exposed to approximately 20 ppm or more of biogenic amine.
The polymer of the invention can be employed by itself as as a sensor to monitor the presence of biogenic amine in food products, particularly meats and fish, on a spot or continuous basis. For example, the polymer can be easily incorporated into common food containers to provide an easily detectable indication of probable spoilage of the food contents within the container.
This invention provides a sensor for detecting the presence of biogenic amine in, on or in association with a food which comprises the polymer and a support structure having a surface, said polymer being attached to said support structure as a coating thereon. In one embodiment the support structure is a plastic sheet, film or tray which is utilized in the packaging of food products. In another embodiment, the support structure is an optical fiber.
The invention also provides a method for detecting food spoilage, comprising:
exposing the polymer of this invention to an environment containing food, e.g., meat or fish, such that fluids (vapors and/or liquids) from the food come in contact with the polymeric sensor, and
detecting any change in color by the polymer, said detected change being indicative of the presence of biogenic amine in, on or in association with said food.
Additional aspects, embodiments and advantages of the present invention will be set forth, in part, in the description that follows, or may be learned from practicing or using the present invention. The objects and advantages may be realized and attained by means of the features and combinations particularly pointed out throughout this description and the appended claims. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention as claimed.